1. Field of the Invention
The present invention relates to electrochemical control of bacterial persister cells and, more particularly, the synergistic effect between weak electric currents and antibiotics on persister cells.
2. Description of the Related Art
Previous studies of persister cells have led to important discoveries that are shifting the paradigm of research in microbiology and antimicrobial therapy. It is now well recognized that subpopulations of bacterial cells in a culture can enter a dormant (non-growing) state that are extremely tolerant to a variety of unrelated stresses such as antibiotics and heat. Such heterogeneity has been reported to exist in even well mixed shake flask cultures at exponential phase. This phenotypic variation can lead to three subpopulations in a given culture including the normal cells, type I persister cells from the stationary inoculums and type II persister cells that are generated during growth. Persister cells are not mutants with drug resistant genes, but rather phenotypic variants of the wild-type strain. Persister cells neither die nor grow in the presence of an antibiotic, and when reinoculated, they grow into a normal culture with a similar percentage of cells as persisters, leading to high antibiotic tolerance.
Although persister cells normally only make up a small portion of the population, they play a critical role in antibiotic tolerance. Most antibiotics inhibit bacteria by targeting growth related cellular activities, e.g., protein, DNA, and cell wall syntheses. They can eliminate the majority of bacterial population by killing the normal cells. For persister cells, however, antibiotics can only repress but not eliminate this subpopulation because persister cells are non-growing dormant cells. Thus, the seeming disadvantage of being dormant in normal environment becomes an advantage for persister cells when being challenged by antibiotics. When the treatment is stopped, some persister cells revert back to normal cells and reestablish the population. Such tolerance leads to reoccurrence of infections and facilitate the development and spread of multidrug resistance through true mutations.
Recent research has demonstrated that persister cell formation increases significantly in stationary-phase cultures and the surface-attached highly hydrated structures known as biofilms. Formed in a dynamic process, mature biofilms typically have mushroom-like structures with cells embedded in a polysaccharide matrix secreted by the bound bacterial cells. Biofilm cells are up to 1000 times more tolerant to antibiotics and disinfectants compared to their planktonic counterparts. Thus, deleterious biofilms cause serious problems such as chronic infections in humans as well as persistent corrosion and equipment failure in industry. Although not completely understood at the molecular level, the biofilm-associated tolerance is due to several factors acting in concert. Bacterial cells in biofilm produce a polysaccharide matrix, which creates a physical barrier that retards or blocks the toxic compounds from reaching the cells. However, protection by the polysaccharide matrix can only partially explain the tolerance because at least some antibiotics can readily penetrate the matrix yet still can not eliminate biofilm cells. Biofilm mode of growth is also associated with changes in bacterial membrane structure and reduction in cell growth rate. The changes in membrane structure could reduce the permeability to toxic compounds, while the reduction in growth rate can lead to higher tolerance to growth-dependent killing by antibiotics. Increasing evidence suggests that the slow growth, especially that associated with persister cells, is the most challenging mechanism for treating chronic infections.